The present invention relates to an image forming apparatus including an image forming apparatus main body such as copying machines and laser beam printers and a sheet post-processing device which is attached to the main body as a different body which applies post processing such as punching and stapling to paper sheets with images formed thereon.
Conventionally, when a post-processing device is mounted on a main body, it is necessary to perform height adjustment so that a sheet path on the main body side and a sheet path on the sheet post-processing device side have an identical height, because paper sheets cannot be fed from the main body to the sheet post-processing device if the sheet path on the main body side and the sheet path on the sheet post-processing device side have a different height.
For example, proposed in Japanese Patent JP3950723B is an image forming apparatus and a sheet post-processing device in which a relay guide plate is placed between the outlet of the image forming apparatus and the inlet of the sheet post-processing device so that paper sheets can be fed from the image forming apparatus to the sheet post-processing device.
However, the image forming apparatus and the sheet post-processing device described in JP3950723B has a problem that unless height adjustment is performed at the time of attaching the post-processing device to the image forming apparatus, the outlet of the image forming apparatus shifts in a vertical direction from a position of the relay guide plate and thereby paper sheets cannot be fed from the image forming apparatus to the sheet post-processing device.
The height adjustment involves time and effort, and therefore in order to make height adjustment unnecessary, a large space needs to be provided between a delivery section of the main body which delivers paper sheets to the sheet post-processing device and a reception section of the sheet post-processing device which receives the paper sheets delivered from the main body.
However, when the large space is formed, the top end of a curled paper sheet S comes into contact with a guide 81 and is widely bent as shown in FIG. 9, and this causes a problem that a jam is prone to occur. Moreover, forming the large space causes a problem that at the time of registering paper sheets, a paper sheet S which is in contact with a nip portion of a roller 82 sags into S shape as shown in FIG. 10 and forms a loop, or sags as shown in FIG. 11 and forms an excessively large loop so that the force of conveying the paper sheet S in an arrow direction is lost.
The term “registering (registration) of the paper sheet” herein refers to the operation of arranging the paper sheet S so that the top end of the paper sheet S is made parallel with the roller 82 by the process in which while one side of the paper sheet S conveyed aslant with respect to roller 82 is brought into contact with the nip portion of the roller 82 to form a loop as shown in FIG. 12, the roller 82 is driven to send out the paper sheet S the moment that other side of the paper sheet S reaches the nip portion.